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Massive storms sweeping in from the Midwest that could affect more than 1 in 5 Americans were expected to bring heavy wind and thunder to the D.C. area Thursday, National Weather Service officials said.

The primary threats include flooding and damaging wind, with a chance for a few isolated tornadoes. The weather service issued a flash flood watch for Thursday until 8 p.m., with high humidity and temperatures in the 90s.

The storm bore down on the Midwest on Wednesday afternoon and evening. For the District and parts of the mid-Atlantic, the big risk was expected to continue and even increase going into Thursday, according to the weather service.

The forecast comes on the heels of a storm in the D.C. area Monday night that resulted in downed trees, scattered power outages and isolated flooding.

Meteorologists warn that the latest line of storms could spawn a derecho, a sudden and severe line of thunderstorms with straight-line winds spanning at least 240 miles.

Thirteen people, including eight in Maryland and Virginia, were killed in a June 2012 derecho that impacted 10 states and the District and traveled more than 700 miles, according to the weather service. Most of the deaths occurred as a result of falling trees. Another 34 people died from heat after the storm, which left 4 million people — more than 2 million in Virginia, Maryland and the District — without power in widespread outages that continued for days.

The National Weather Service said it’s too early to tell whether the downpour might be as destructive as last year’s storm, but the setup of this storm is “entirely different.”

“There’s a line of storms that will come in from the Midwest again, but the thunderstorm threat looks much more like Monday’s heavy rain,” weather service hydrologist Jason Elliott said.

There were no immediate reports of fatalities or major damage Wednesday evening as the massive line of storms began rolling through the Midwest.

Several small tornadoes were reported in parts of Iowa and in Illinois, and there were more than 1,000 power outages in northern Iowa.

More than 120 flights at O’Hare International Airport in Chicago were canceled. Several major events, including a Chicago White Sox baseball game, also were canceled.

All told, the area the weather service considers to be under heightened risk of dangerous weather — including hail, lightning and tree-toppling winds — includes 74.7 million people in 19 states from Iowa to Maryland.

In the event of a tornado warning, officials recommended taking shelter inside a sturdy structure immediately and staying toward the center of the building in a small room.